brachytherapy sources, dosimetry, and quality assurance for … · 2014-07-03 · brachytherapy...

Jeffrey F. Williamson, Ph.D.

Brachytherapy Sources, Dosimetry, and Quality Assurance for 192Ir HDR

Brachytherapy

AAPM Therapy Review Course19 July 2014

Virginia Commonwealth UniversityVCU Radiation Oncology

Learning Objectives: Focus on 192Ir HDR

• Radionuclide properties and their clinical applications• Brachytherapy quality assurance strategies

– `TG-100 approach to patient-specific HDR QA

• Brachytherapy dosimetry principles and practices– Measurement of brachytherapy source strength– Evaluation of dose rates around individual brachytherapy

sources– Implications for QA program

• Dr. Thomadsen’s Topic: brachytherapy treatment planning- the art of arranging multiple sources in various clinical settings

How do physical radionuclide properties determine their clinical applications?

Source properties: energy, half-life, specific activity• Dose rate:

– High: > 12 Gy/h– Low: 0.3-1.5 Gy/h– UltraLow: <0.2 Gy/h

• Mode of delivery: Interstitial, intracavitary, surface• Dose control mode: temporary, permanent• Source transport mode: hot loaded, manually

afterloaded, remotely afterloaded• We will consider high dose rate (HDR), temporary,

remotely afterloading implants

Understand this Table

Williamson, Li, and Brenner, PPRO 6th ed

Influence of Photon Energy On absolute Dose Rate

• >200 keV, all sources have same DRC, regardless of medium

• <100 keV, photo effect induces up to two-fold heterogeneity corrections0.20

0.40

0.60

0.80

1.00

1.20

1.40

101 102 103

Dose-Rate Constant in Fat and Water Media

Dose to fat in fat medium

Dose to water in water mediumDos

e R

ate

per u

nit A

ir-K

erm

a St

reng

th

Photon Energy (keV)

198Au,192Ir

137Cs

60Co, 226Ra

125I

241Am

Dose rate in medium at 1 cm Dose-Rate ConstantAir-kerma rate in free space at 1 cm

Effect of Energy on Penetration

• Above 200 keV: All photon emitters have same depth dose regardless of medium

0.00

0.01

0.02

0.03

0.04

0.05

0.06

101 102 103

Dose (5 cm)/Dose(1 cm)

Dose to Water in Water Medium

Dose to Fat in Fat Medium

Dos

e at

5 c

m/D

ose

at 1

cm

Photon Energy (keV)

198Au,192Ir

137Cs

60Co, 226Ra

125I

241Am

21 0.045

Low Dose-Rate Intracavitary Brachytherapy• Cs-137 sources:

– Ceramic core (low toxicity)– 662 keV photons (radiation

exposure management )– 30 year half life (10 year life)– Low specific activity (LDR only)

3 . 9

2 . 3

1 3 .8

1 .1

2 0 .0

3 .0 5

3 M Model 6 5 0 0 / 61 9 7 ? - 1 9 9 1

High Dose-Rate BrachytherapySingle-Stepping source remote afterloading

• Ir-192: Half life = 73.8 days, mean energy = 397 keV• Very high specific activity• HDR Ir-192 source: SK = 4.08 x104 Gy m2 h-1

• Ir-192 also used for LDR interstitial implants

Trans-rectal Ultrasound-Guided Perineal Permanent Prostate Implant

Ultrasound image

• Patient’s tissues effectively shield staff and public from exposure

Isotope Half-Life Energy Dose rate

I-125 59.6 days 28 keV 7 cGy/hPd-103 17.0 22 21 Cs-131 9.7 30 36 Au-198 2.7 412 105

4.5 mm x 0.8 mm titanium-clad seeds

High Dose-Rate Brachytherapy

• Greater potential for high-severity medical errors – degrees of freedom (dwell times & positions) more

error pathways– Insertion, planning and delivery in few hours stress on

staff– Source detachment 20 mm diameter sphere receives

dose of 750 cGy/min

Quality Assurance Taxonomy• Quality Assurance of Devices (TG-56):

– Do applicators, afterloaders, sources, planning systems work properly?

– Commissioning/acceptance testing» Identify malfunctions/test users’ beliefs» Transform physicist into expert user» Integrate device into clinical program

– Periodic QA protocols» Device still function within specs? Users’ beliefs still valid?

• Patient-specific QA or “Process” QA (TG-56 for LDR; TG-59 for HDR): none for Image guided BTx– During individual patient treatments: prevent treatment

delivery errors and high risk scenarios

Quality Assurance Fundamentals• Accurately deliver dose distribution desired by

radiation oncologist– Clinical intent correctly translated into prescribed dose

and normal tissue constraints– spatial-temporal accuracy: correct sources placed in

prescribed location for prescribed time– accurate dose delivery

• Specific endpoints– Positional accuracy (± 2 mm)– Temporal (timer) accuracy (± 2%)– Dose delivery accuracy (± 2-20%)– Safety: Patient, staff, public and institution

Safety Endpoints• Protect staff and public

– Uncontrolled areas: < 0.02 mSv/h regardless of occupancy (10 CFR part 20)

– General public: < 1 mSv/y to any person– Staff: < 5 mSv/y per ALARA

• Protect patient from catastrophic errors– Verify all critical “decision points”– Verify interlocks/ error detection systems– Emergency and error recovery procedures

• Institutional protection– Complete/accurate records– Adhere to/document compliance to CMMS and 10 CFR 35

14

Example: Risk-Informed QM Formulation for Brachytherapy

• Scenario: Accelerated partial breast irradiation using multi-catheter balloon HDR brachytherapy applicator

– CT-based evaluation and planning– Multicatheter balloon applicator, e.g., Contura– Automated plan transfer but not full EMR charting

• “Standard” QA practice– Fixed, one-size-fits-all prescriptive QC protocols – Strong physics-centric focus on device QA

• Risk-informed QM practice: TG-100– Multidisciplinary– Focused on processes not devices– Uses formal risk analysis tools to create customized QMP

15

Image-Guided Balloon Catheter PlacementAccelerated Partial Breast: MammoSite HDR BTx

• gg

Intraoperative Ultrasound

Visualize LumpectomyCavity: Select Approach

Assess conformality

Intraop/PostOp CT

Assess conformality

Assess SymmetryD. Arthur, VCU

Assess Skin Distance

16

Contura Multi-Cath Balloon Applicator Mismatch errors

• ee

Asymmetric loading to

spare skin and chestwall

TG-100 Risk Analysis Steps

• Steps1. Define process by creating a process map2. Failure modes and effects analysis (FMEA): Identify threats

to success (failure modes) and rank according to risk3. Fault-tree Analysis (FTA): Propagation of failures through

system and placement of QM interventions4. Develop QA or QC interventions to mitigate risk

17

TG-100 Risk Analysis Steps• Process Map: Step 1

– Delineate and then understand the steps in the process to be evaluated

– Visual illustration of the physical and temporal relationships between the different steps of a process

– Demonstrates the flow of these steps from process start to end

• prospective risk analysis for hypothetical clinical process modeled on VCU and UW-Madison processes

– Assumes NO QA or QC checks– Partial automation of EMR and data transfer– 4 physicists did ranking (Ibbott, Thomadsen, Mutic, JFW)

18

19

Breast Brachytherapy Process Map

TG-100 Risk AnalysisStep 2 FMEA

• Step 2a: For each process step, ask the following questions

– What could possibly go wrong ? (enumerate/ describe failure modes)

– How could that happen? (what are possible causes of FM?)– What effect would such an undetected failure have? (Potential

impact on quality)• Step 2b: Assess risk of FM by estimating O, S, and P • Present analysis: 96 Failure Modes

20

Process tree

Sub-process #1

Sub-process #17

Sub-process #19

Step #4Step #1 Step #j

Failure mode #2Failure mode #1 Failure mode #k

Causes of failure #6Causes of failure #1 Causes of failure #m

Effects of failure #4Effects of failure #1 Effects of failure #n

Step 2a: enumerate FMEA Failure Modes

22

Assess Risk Posed by Each FMStep 2b

• For each subprocess, enumerate the possible scenarios, i.e., Failure Modes (FM), that could lead an unsuccessful treatment: 96 FMs

– Identify causes and effect on process outcome• Assess risk to successful outcome posed by each FM

assuming no QA

– Assign O,S, and P a value from 1-10– 4 Observers: Ibbott, Mutic, Williamson, Thomadsen– Significant additions/modifications by JFW

• Reorder list in terms of descending RPN

Likelihood of Severity of Likelihood ErrorRisk occurrence consequences Detected

O S P Risk Proba

No

bility Number R S P

t

PN O

TG-100 FMEA Rating Scales

• ff

23

24

Co mb ined

r M a jor Proce sses Step P ote nti a l Fa i lure M ode s

Pote ntia l Ca use s of Fa il ure JFW Comme nts a nd de sc ripti ve sce na ri o

P ote nti a l Effe c ts of Fa ilur e

AV G O AV G S AV G D Avg RPN

Im ag in g a nd d ia gno sis

RO revie ws EM R prior t o RO cons ult

Me d O nc or Surge on con sulta tion mis in terprets or mis rep res ent s p rim ary clinical fin dings ( im a ging stu dies, p ath rep ort s, et c); in corre ctly st ages pat ient , a nd rec om me nds BCT and AP BI for pat ient t hat is n ot app rop riat e can didat e

RO ba ses Tx recom m end atio n o n se cond ary M D re port rat her t han revie wing prima ry clinical fin dings

a nd disco vering t he ups tre am erro r

Up stream phys ician error p ote ntia lly disc overable by Ra d

On c s in ce prim ary c lin ical d ata is availab le W e s hould re com m end t hat t he RO perform s th eir dut ie s

d ilige ntly .

wron g/ve ry wrong do se dist ribut ion 5.00 8. 25 5.5 0 269 .3

Im ag in g a nd d ia gno sis

RO revie ws EM R prior t o RO cons ult

pat h o r biom ark er rep ort s is inc orrect du e to m is labeling of surgical spec ime n o r biom arker re port . Hen ce pat ient is und ers tag ed and in app rop riat ely o ffe red BCS by Me d On c and Surgeo n

RO rec om me nda tion for AP BI is fu lly co nsist ent w ith prio r EM R

An error n ot eas ily disc overable by Ra d On c B ase d o n t he wo rse cas e.

V ery wro ng dose 4.25 8. 75 8.2 5 309 .5

Pa tien t d ata bas e informa tion

E nt ry of p atie nt data in RO E MR o r writ te n

chart

Inco rrect pa tient ID dat a Doc um entat io n e rror

W ron g p atie nt ID le ading m isfilin g of d em ographic an d c lin ic al d ata from h osp it al DB; ide ntifica tion of wrong pa tient

V ery wro ng dose 3.00 8. 75 2.7 5 7 0.0

P at ie nt Da tab ase Informa tion

E nt ry of p atie nt data in RO E MR o r writ te n

chart

Co rrect pa tient ID da ta but clinica l

fin dings/ ima ges fro m wrong pat ient loa ded

in to RO EM R

Om is sion in e ntry, inco mp lete pat ient hist ory

Incorre ct clinical fin dings le ads to faulty de cision to t rea t o r downst re am pe er-revie w correc tion

V ery wro ng dose 5.00 7. 75 3.7 5 154 .5

Co nsu lt at io n a nd de cision to t rea t

De cision of t re atm en t t ech nique an d p rotoco l

Clinica lly ina ppropria te pa tient s elected for

A PB I

m isint erp ret atin g o f clinica l fin dings inco mp lete H&P

Eve n t hou gh upst re am clinica l dat a a re co rre ct, E rror by RO ass essin g ind icat io ns and con tra indica tions t o A PB I., e .g. , SL N+ with Su rg unt re ate d a xilla . RO m is rep res ent s o r negle cts ke y fin ding and offers in approp riat e treat me nt plan to pa tien t

V ery wro ng dose 4.25 7. 75 7.7 5 252 .8

Con sulta tion an d dec ision to treat or im aging /diag nos is

De cision of t re atm en t t ech nique an d p rotoco l

or im agin g/d ia gno sis

p atie nt wit h radio gra phically to o

large or clos ed serom a c avity se le cte d

RO error in inte rpretin g im agin g stu dies; ina ppropriate im aging use d; or poo r im ag ing qualit y

JFW : New failure mo de

V ery wro ng dose if not de tec ted ; mo re lik ely

m ajor inco nven ie nce or in fe ctio n from

un nece ssa ry invas ive pro cedu re

4.75 6. 25 4.7 5 140 .3

First 6 Failure Modes

25• Red entries: New JFW FMs or modified RPN

8 Highest Risk FMs

Fault Tree Analysis and Designing QM interventionsSteps 3 and 4

• Step 3: Create Fault Trees (optional)– Time consuming: Limit FTA to selected FMs– Visualize interactions between FMs possibly in different

process tree branches– JFW: helped me refine list of FMs and scenarios

• Step 4: Design QM intervention– Rank FMs according decreasing risk and severity– Mark high RPN/S FMs on fault and process trees– FTA guides optimal placement of intervention– Design intervention: balance cost, specificity, sensitivity and

benefit

26

Fault Tree AnalysisStep 3: TG100 risk analysis methodology

• FTA compliments process tree• Leftmost box is the failure (error)

• Each daughter node is a FM that could cause the error

• Works backwards in time (to the right) until root cause is reached

• Models propagation of error through system

27

• ‘OR’ means error occurs if any one of antecedent FMs occurs• ‘AND’ means all antecedent FM’s must be realized for error to

occur

• No QA/QC assumed

• Relevant FMs scattered across at least 4 process tree branches

• Interactions

28

Program treatment unit

failure

Wrong data file imported

Inconsistency between treatment

program and default operating

parameters

Software failure

InattentionOr

Or

Dose in wrong location due to source position

Units length distance is incorrect

Unit step size inaccurate

Applicator in wrong location

Or

Single or multiple catheter failure

Catheter trajectory inaccurately

localized

Incorrect catheter number asssigned

Wrong catheter slice images

Inadequately trained personal

Poor labeling on photographs

Poor image quality

OrOr

Or

Dwell position construction

failure

Distal-most dwell location

inaccurately digitized

Treatment length incorrect (wrong

transfer tube length, wrong

sounding information, wrong

dwell spacing)


Poor images

Default distances used

Equipment failure

Or

Or

Or

48

Or

Systematic offset Commissioning

failure

Catheter localization

failure

Wrong catheter position Marked

Catheter indicators not inserted fully

Or

Or

Non-Positional Failure Modes

Post-procedure CT imaging

error

Sounding measurement error

Channel numbering error: marking or

recording

Channel and applicator numbers not matching when connecting transfer tubes to applicator

Wrong length transfer tube

OrConnect transfer

tubes to applicator failure


Poor quality/incomplete

images

Initial treatment failure

Or

Treatment planning

Error

Channel Mismatch

Or


Incorrect Catheter Polar rotation

26

25

26

25

23,24

51

49

50

46

52

52 53

73

75

72, 70, 71

86

76

Source Positioning ErrorFault Tree

Positional Accuracy• Each active dwell position delivered to correct

location in correct applicator within ± 2 mm (TG56)• “Correct” Designated treatment positions in

plan coincide with radioactive source center during delivery

– actual source center = position of radiographic dummy marker

– HDR unit ejects correct cable length into programmed channel

– Structured set of tests for each applicator type» transfer tube length» HDR source-dummy seed coincidence

Positional Accuracy: HDR BTx

Dwell position localized in CT

Source center accurately transported to planned position

• Error types– Channel mismatch– Incorrect Tx length– Incorrect step length

• Top level causes– Post procedure imaging

error– Tx Planning error– Error in treatment setup

or device programming

31

Dose in wrong location due to source position




Or

Single or multiplcatheter failure

Dwell positionconstruction

failure

O

Or



error


Treatment planning

Error

Channel Mismatch

Or

Source Positioning ErrorFault Tree

• Incorrect information /poor images Dwell position programming error

– Channel numbering or documentation– Catheter length measurement– Imaging performed with incorrect marker position

• QM interventions– QC: second therapist assists with measurements– QA: Independent check of localization data before patient leaves imaging

suite

32


failure



Or

Or



error



recording


images

And Physicist/dosimetrist

check images failure

Adequate QM program for planning and afterloader

systems

And

Assisting therapist misses errors

26

25

26

25

23,24

Post-procedure CT imaging Localization Errors

33

Post-Procedure ImagingLocalization steps

34

Localization FMs:Treatment Planning

• Catheter trajectory delineation error– Dwell 1 length error

» Systematic positional offset error– Dwell position digitization error

• channel mismatch error



localized





Poor image quality

Or Or

Or


failure






dwell spacing)


Poor images


Equipment failure

Or

Or

Or

48

Or


failure



Or

Or




images

Initial treatment Operator check

And

Physicist check plan failure


systems

Treatment planning

Error

26

25

26

23,24

51

49

50

46

52

52 53

Treatment length

Dwell positions

35

Example: Systematic Offset Error• Systematic source positioning

error: caused by invalid treatment length estimation protocol

• Varian “quick connect” indexer interface

– 14 mm offset compared to standard transfer tube connector with usual transfer tube-applicator combination length measurement

– No Software offset or hardware interlock initially provided

36

TG-56 Structured HDR Positional Accuracy Tests

dwell 1 (1500 mm) position

indexer reference L1 = 0.0

transfer tube

Tube Length: L (1218 mm)

t

Fully Inserted Radiographic Marker

L RM

transfer tube L1 (programmed length) = 1500

mm

Tube Length: L (1218 mm)

t

Radioactive Source Programmed to 1500 mm

PDR Head

Compare for

Coincidence

0"O dcalibrated dummy ribbon

d : Dummy Insertion Depth

iApplicator Orfice

37

Mitigating RTP Localization FMs

• Implement well-defined, rigidly followed procedures: – Adequate patient volume

• QC: use only one transfer tube length & use equi-length catheters

• QA: Final physics plan review focus on dwell position



localized





Poor image quality

Or Or

Or


failure






dwell spacing)


Poor images


Equipment failure

Or

Or

Or

48

Or


failure



Or

Or




images

Initial treatment Operator check

And



systems

Treatment planning

Error

26

25

26

23,24

51

49

50

46

52

52 53

• Adequate device QA: – Maintain image quality– Eliminate offsets and

incorrect default parameters

– Consistency of procedure with device function

• QA/QC in red• Adequate device QA

protocol• Written procedures

– Redundancy– Uniformity– Patient volume– Training to ensure

compliance• Physics Checks

– Simulation– Tx plan– Setup/RAL

programming

38

Program treatment unit

failure

Wrong data file imported

Inconsistency between treatment

program and default operating

parameters

Software failure

InnatentionOr

Or




Or



localized





Poor image quality

Or Or

Or


failure






dwell spacing)


Poor images


Equipment failure

Or

Or

Or

48

Or


failure


failure



Or

Or



error



recording

Channel and applicator numbers not matching when connecting transfer tubes to applicator

Wrong length transfer tube

OrConnect transfer

tubes to applicator failure



images


Or

And

Operator check failure: imported

parameters vs. plan

And


And Physicist/dosimetrist

check images failure


systems

Treatment planning

Error

Failure: Physicist & MD

final setup check

And

Channel Mismatch

Or


And

Assisting therapist misses errors

Incorrect Catheter Polar rotation

26

25

26

25

23,24

51

49

50

46

52

52 53

73

75

72, 70, 71

86

76

Mitigating Source Positioning Errors

Dose Calculation FMs

• Wrong source, decay correction, or units• Wrong dosimetric parameters• Program malfunction• Input data error

39

40

Conclusions: formal risk analysis• Process mapping and FMEA advantages

– Focuses attention on process as well as device failures– Provides a vehicle for team to work collaboratively to

» better understand the process » Appreciate each other’s vulnerabilities » buy into core QM/QI values

– Most expert member gets to fix FM– Promotes clinical process uniformity so that desired process-

step outcomes get internalized– Better understanding of device-process interactions helps

physicist prioritize device QA• Downsides

– Resource intensive to build/use FMEA expertise– Not a mechanical, one-size-fits-all prescriptive approach:

requires judgment and individualization

41

Dose Delivery Accuracy• Algorithmic Accuracy (±2%)

– Given a known input, the calculated dose agrees with algorithm specifications

• Physical Accuracy (±5%)– Given perfectly positioned source and point of interest with no

error in dwell time delivery thenDose Delivered = Calculated Dose

• Clinical Accuracy (±10-20%)– Actual dose to patient = calculated dose– Includes errors due to: source targeting accuracy, organ

delineation error, seed migration, tissue deformation

How is strength of clinical brachytherapy sources determined?

• Answer: In terms of air-kerma rate on transverse axis for all photon emitters

2004 AAPM Definition of Air-Kerma Strength

K (d) is air-kerma rate in vacuo due to photons of energy > ( 5 keV), d L Cutoff designed to exclude low-energy contaminant radiation

22 1 2 1KS [ Gy m h cGy cm h U = K d ](d)

NIST Primary Kair and SK Standards

• Carbon-walled spherical cavity ionization chambers– Realizes air-kerma standards for Cs-137 and Co-60 for

teletherapy & brachytherapy and for Ir-192 LDR seeds

• Primary Standard: Maintained by National Institutes of Standards and Technology (NIST)– All other instruments calibrated against it

– Measures absolute amount of a quantity in terms of time, mass, charge, length

Source Calibration Options for HDR RAL• TG-56: in-air method as interim secondary standard• For quarterly calibration end users can

– Duplicate interpolative in-air calibration technique OR– Use HDR well chamber calibrated against in-air method by ADCL

• TG-56 recommends independent tertiary standard as redundant check

calibrated re-entrant well chamber

Interim secondary standard using cavity ion chamber

Traceability and AAPM Recommendations• ADCL: AAPM-accredited secondary lab which can

calibrate a user’s source against NIST standards• Directly traceable calibration: source/instrument

has NIST or ADCL calibration• Secondarily traceable: source or instrument

intercompared to a source with ‘directly traceable’ calibration.

• AAPM recommendations (TG 56 and 40):– All clinical sources should have secondarily traceable

calibrations– Each user should verify vendor calibrations with

secondarily traceable SK measurements

How are dose rates around individual sources calculated?

• By inferring dose rate to surrounding medium from measured SK of the source

• Classical dose calculation (1940-present)– Dose model parameters independent of source geometry– Point source model and Sievert integral

• Quantitative Dosimetry (1980- present) – Source model-specific dosimetry parameters derived from

Monte Carlo simulation and/or TLD measurement– TG-43 protocol: standardized table-based single-source

dose-rate calculation using MC and TLD data • For 137Cs and 192Ir, classical and quantitative

approaches are equivalent on transverse axis

AAPM Dosimetric Prerequisites for Routine Clinical Use of > 50 keV Sources

Li Med. Phys. 34:37 (2007)

• SK values used for planning shall be secondarily traceable to NIST WAFAC calibrations

– Annual intercomparisons between vendor, NIST, and ADCLs• Independent published Monte Carlo and experimental

dose-rate distributions– For ‘conventional’ 137Cs and 192Ir, one determination sufficient

• Compliant sources listed on AAPM/RPC Registry

AAPM High Energy Brachytherapy Dosimetry (HEBD) Report

Med Phys 2012

HEBD Report contents

• Extension of TG-43 formalism to higher energy and extended sources

• Guidelines for Monte Carlo determination of dose-rate distributions

• Consensus dose distributions: TG-43 parameters and away-along tables

– 14 HDR and PDR 192Ir sources + 2 LDR seeds– 2 60Co HDR sources– 3 GYN 137Cs tubes

• Nearly all datasets are Monte Carlo based

HDR and PDR Sources with HEBD Consensus Data

AAPM Revised 2-D Formalism1°

r

LL

LK

G (r, )D(r, ) S F(r, ) g (r)G (1 cm, / 2)

L 12 2

2P

if 0 line-source Lrsin approximation

r L / 4 if 0

point-source

approximation

for x = L G (r, )

for x = P G (r, ) r

Where L = effective active length of sourceHEBD: Only GL recognized

Starting Point: discrete grid of dose rates measured by TLD or

calculated by Monte Carlo

Transverse Axis Measurement Phantom

wat

K

wat

K

r = 1 cmD at / 2S

where D = TLD measurement S = NIST-traceable measurement

90 o

60 o

45 o

20 o

0 o

120 o

135 o

160 o

180 o

Polar Dose Profile Measurement Phantom

• HEBD: Assume full scatter 40 cm radius liquid water phantom

HEBD Consensus L values: 192Ir HDR Sources

1wat 0 0wat

en air 0K,N99 0

D r = 1 cm, / 2 2 tan L / 2r( / ) T(r )

S Lr

TG 43 Radial Dose Function: gL(r)

• gX(r) = dimensionless radial dose function Describes transverse-axis dose Fall-off

• GL-Factor suppresses dose variation due to inverse square-law fall-off

LL

L

D(r, /2) G (1 cm, /2)D(1 cm, /2) G (r, /2)

g (r)

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

g L(r

)r (cm)

Radial dose functionmHDR-v2

Daskalov, Löffler &WilliamsonDaskalov&Williamsonunbounded

microSelectron V2 Source

LL

LK

G (r, )D(r, ) S F(r, ) g (r)G (1 cm, / 2)

Task Group 43 2-D Anisotropy Function: F(r,)• F(r,) = dimensionless

anisotropy function– Describes angular dose

variation at fixed distance– GL suppresses inverse-square

dose variation

LL

D(r, ) G (r, /2)D(r, /2) G (r, )

F(r, )

1°

r

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

F(r,

)

Angle (degree)

Anisotropy functionmHDR-v2

0.25 cm0.5 cm0.75 cm1 cm1.5 cm23 cm4 cm5 cm6 cm (extrapolated)8 cm (extrapolated)10 cm (extrapolated)

microSelectron V2 Source

LL

LK

G (r, )D(r, ) S F(r, ) g (r)G (1 cm, / 2)

TG-43 Dose-Calculation ‘Algorithm’• TG-43-HEBD starts with a discrete grid of Monte Carlo

dose rates• F(r,), g(r ), and an(r ) table entries correspond to

MC calculation points• What does RTP do at arbitrary point (r,)?:

– Finds g(r1) and g(r2), etc. at nearest neighbor points– Calculates g(r ), etc., by bi-linear interpolation

– Calculate exact G(r,) and obtain D(r,) from TG-43 equation

• QM: compare RTP single-source dose rates with manual TG-43 or HEBD away-along calcs

12 1 1

2 1

r rg(r) g(r ) g(r ) g(r )r r

Monte Carlo Dosimetry Techniques

• Simulate photon histories for source embedded in a water phantom and a free-air calibration range

• Quantities calculated

Monte Carlo Validation192Ir Brachytherapy

• TLD and diode dose measurements show good agreement with Monte Carlo

• Uncertainties– Experimental: >5%– Monte Carlo: <2%

• HEBD: all consensus data based on MC

– MC: better range and spatial resolution

Kirov/Williamson Med Phys 2005

Importance of secondary electron transport for 192Ir

Ballester, Med Phys 2009

• <1.5 mm distance, CPE breaks down and coupled photon-electron MC is needed to achieve 2% accuracy. Elsewhere Dose Kerma

Bebig HDR Source: F(1 cm,) • Very small differences in

anisotropy function for similar geometry sources

• Various MC codes (Penelope, PTRAN, EGSnrc, MCNP) all agree closely

Granero Radiother Oncol 2005

‘Classical’ Dose Calculation Model

medK en air

med 2S ( / )D (r) T(r)

r

• No radiative loss and CPE

meden air

ratio of mass energy Where ( / ) absorption coefficients

• Then

medmed air en airD K ( / )

Tissue Attenuation Factor, T(r)

• Describes competition between primary attenuation and scatter buildup

• T(r ) = 1 ± 0.05 for r < 5 cm when E >200 keV

• Often derived from 1-D transport calculations

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 5 10 15

169Yb: Type 8 Seed

125I: Model 6702

192Ir: MicroSelectron HDR137Cs

198Au

60Co

Tiss

ue A

ttenu

atio

n Fa

ctor

, T(r

)

Distance (cm)

wat

wat

D (r) in WaterT(r) Tissue Attenuation FactorD (r) in Vacuum

Apparent Activity: Aapp

Aapp = activity of hypothetical unfiltered point source of same radionuclide that gives same SK as the given source

• Units: mCi, Ci, Bq, or MBq• Applicable to all photon emitting radionuclidesCommonly applied to I-125, Pd-103, & HDR Ir-192

• Uses: regulatory compliance and for interstitial implant dosimetry

X

X

medapp 2

app K

f T(r)D(r) A

r

WA S e

Sievert Filtered Line-Source Integral1D Path-length Model

• = effective filtration• Accurate on transverse axis for

all sources > 100 keV• Cs-137 tubes: accurately models

2D anisotropy• Ir-192: >10% errors in 2D

anisotropy function

2K

1

wat ten t secair/ e

D(x,y) S e T(x sec ) dL x

Y

XL

P(r,) = P(x,y)

r

t

L

'

x

y

r'

1D Pathlength Model vs. Monte CarlomicroSelectron ‘classic’ HDR 192Ir source

• %RMS error = 6.9% Williamson IJROBP 1996

67

Dose Calculation QA • Planning system algorithm: numerical accuracy

– Algorithm output vs. independent calculation• Physical accuracy

– Algorithm output vs. Monte Carlo or measurement – SK calibration accuracy

• Clinical accuracy– Image identification and display– Constructing 3D images from slices– Forming bit-map or surface-mesh structures from contour

stacks; ray tracing, etc.– DVH/plan evaluation metric accuracy– Source/applicator reconstruction from CT or radiographs

• System integration tests: dry runs and end-to-end testing on phantoms

Avoiding patient-specific random errors

Physicist Pre-Tx HDR Plan Review Checklist

• Main TG-59 strategy for intercepting/correcting major errors

• Comprehensive check: consistency & correctness

– Prescription, Clinical policies, localization images, implant diagram, plan

• Physicist: avoid compromising independence

– Train dosimetrist to do planning

Patient-Specific Manual Dose check

• Independently measure CTV dimensions, assess total SK

• Usually, 5%-10% agreement with RTP

0.10

1.00

1.0 10.0 100.0

192Ir: Manchester 125I: Memorial125I: Yu 103Pd Memorial103Pd: Yu

D

ose

Rat

e/A

ir-K

erm

a St

reng

th:

cG

y/(

Gy

m2 )

Target Volume (cm3)

3.49 0.667 2.57 0.733 2.22 0.683 2.71 0.852 1.88 0.7402.00

0.50

0.20

0.05

4.00

3.0 30.0

KD S V

brachytherapy sources, dosimetry, and quality assurance for … · 2014-07-03 · brachytherapy...

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